Dear Vikram, I can only give you a general diagnosis since I have no knowledge of the medium, the electroactive species and the electrodes involved in your experiment.

The absence of a reduction peak suggests to me that the oxidation product is consumed by some chemical or physicochemical reaction, with the result that there is no reactant for the reverse, re-reduction reaction to proceed. The best solution would then be to inhibit this chemical reaction, or manipulate the medium to ensure that no such  coupled chemical reaction takes place. Alternatively, it might be possible to shorten the time scale of your measurement (by increasing the scan rate) so that at least a fraction of the oxidation product 'survives' to be reduced back to the reduced species: try to increase the scan rate to lessen the dominance of the chemical reaction, a small reduction peak might appear.

If the above suggestions fail AND if the electrochemical process is known to be fully reversible, then you could assume that the oxidation (observed) and reduction (absent) peaks are positioned on either side of the formal electrode potential so that

E1/20= (Eox + Ered)/2

and by further assuming that the separation between Eox and Ered is 59 mV / n for an n-electron couple, you could calculate E1/20.

Best of luck

Thank you @François L L Muller.

Your explainations have given me some new ideas.